The predominant ways of heating residential, commercial, and industrial space using a central heat source are few in number, comparatively old and very well-known. One way is to produce steam in a boiler, which is usually fired with a fossil fuel (gas, oil or coal), and distribute the steam through pipes to radiators located in selected places in the space. Central steam heating systems are no longer widely used in new construction, inasmuch as they are relatively costly to install, present maintenance problems due to scale buildup, are difficult to control effectively and require large, heavy and often unattractive radiators. A second way is to produce hot water in a furnace and pump it through convectors. As compared to steam systems, circulating hot water has the advantages of being of lower initial cost, presenting fewer maintenance problems, being easier to control (because of lower heat storage in the convectors), using generally smaller and less visually and physically intrusive convectors, and being compatible with air-cooling using the same convectors and the same piping or separate piping. A third way is to heat return air from the space in a furnace or a heat pump and circulate the heated air back to the space through ductwork. Forced air systems are relatively inexpensive to install but require comparatively large ducts to keep air velocities low and thereby minimize noise and reduce distribution losses due to highly turbulent air flow. Also, like circulating water systems forced air central heating may incorporate air-conditioning (air-cooling). Devices for air cleaning and humidification can readily be added to a forced air heating/cooling system.
While there have over the years been many improvements and new developments in various components of known central space heating systems, such as more efficient fuel burners and furnace heat exchangers, solar heat sources, heat pumps, and the like, the basic systems (steam, circulating hot water, and forced air) have existed without change in principle for perhaps a century or longer. Moreover, central heating is much more widely used than room heating, largely for economic reasons. Room-by-room heat pumps and baseboard electric heaters, the main devices for room heating, are more costly to operate and are, therefore, widely used only in residences in warmer climates and in vacation residences where heating is needed relatively infrequently.